Surface treatment of aluminum and aluminum alloys



Patented Oct. 19, 1937 PATENT OFFICE SURFACE TREATMENT OF ALUMINUM AND ALUMINUM ALLOYS Alfred Norman Douglas Pullen, London, England assignor to The British Aluminum Company Limited, London, England No Drawing. Application December 4, 1935, Se-

rial No. 52,932. In Great Britain December 17,

9 Claims.

This invention relates to the surface treatment of aluminum and aluminum alloys and more particularly to treatment directed to the improvement of the specular reflectivity of such surfaces.

Sheet aluminum as produced by the normal rolling process has an average specular reflectivity for light of about 65 per cent.

The reflectivity may be increased by processes such as polishing up to a figure of 75 per cent. to 80 per cent.'but on exposure to air this enhanced reflecting power decreases rapidly until a figure equivalent to that for normal rolled sheet is reached.

The object of the present invention is the provision of a method and means by which the specular reflectivity of an aluminum or aluminum alloy surface may be increased very considerably in a cheap, eflicient and permanent manner.

The present invention consists in a process for increasing the specular reflectivity of the surface of an aluminum or aluminum alloy article which comprises immersing the previously degreased article in an aqueous solution of sodium carbonate and tri-sodium orthophosphate to remove the normal oxide film, then immediately thereafter in the same solution subjecting the article to anodic treatment by direct current at a moderately high current density for a short time to stop the attack of the solution upon the article and continuing the anodic treatment by direct current at a lower current density, and, after washing in water, subjecting the article to further electrolytic treatment in a strong aqueous solution of sodium bisulphate.

The invention may be carried into effect in one way by way of example as follows:-

A piece of commercially pure rolled aluminum sheet is first degreased by suitable means such as cleaning in petrol, then, if necessary, it is buffed or otherwise polished mechanically. It is then given a. chemico-electrical treatment in a bath having the following composition and under the following conditions.

Electrolyte 15 per cent. by weight of sodium carbonate, per cent. by weight of trisodium ortho-phosphate in water. The solution to be heated to 80 0.:5.

Procedure The metal article is immersed in the bath with the result that solution of the metal by the alkaline electrolyte commences almost immediately. As soon as uniform attack is observed, approximately after seconds immersion, direct current at 5-6 volts is switched on, the aluminum being the anode, the current density applied being of the order of 30 amperes per square foot. An iron or steel cathode is used and procedure in the bath follows electro-plating practice to some extent,

e. g., the cathode is made to conform roughly to the anodic surface.

It is of advantage to have a bath which is of considerably larger dimensions than the electrodes. At the end of a short period, about 30 seconds, the current density automatically falls to about half the first value whilst the voltage is maintained approximately at the figure mentioned. The treatment is continued under these conditions for about 10 minutes. The article is then removed from the bath with the current still on but preferably reduced to a small value by suitable means, and immediately washed in clean water. The surface of the metal, particularly that one facing the cathode, has now a very bright appearance and it will be found that if the original specular reflectivity value was, say, 65 per cent, it has now been raised to perhaps 80 per cent. Besides the general bright appearance, it will be noticed that the surface of the metal now carries a thin film, presumably of aluminum oxide which shows brilliant interference colours.

For the following part of the treatment, that is to say, the provision of a protective oxide film, the metal is again subjected to an anodic process in another electrolyte, the composition of which and the conditions of operation being as follows Electrolyte Aqueous solution containing 25 per cent. by weight of sodium bisulphate.

Voltage 6-12 volts direct current, the aluminum being the anode.

Temperature of electrolyte Time of treatment 10-15 minutes.

The current density is of the order of 5 amperes per square foot.

Whilst a dual process has been described in the foregoing example, it is possible to carry out the process in two separate steps, a brightening and a final protective step which latter may, if necessary, be carried out by suitable known anodic processes.

General The figures given in the foregoing example are capable of modification without departing from the scope ofthe invention.

For example, for the first part of the treatment the composition of the electrolyte may include all strong alkaline electrolytes having a pH value of over 10, containing an ortho-phosphoric acid radicle corresponding in quantity to that contained in approximately 3 to 9 per cent, by weight of trisodium ortho-phosphate, and with akalinities due to the hydroxides or carbonates of the alkali metals such as sodium, potassium, lithium, with or without the addition of alkali metal salts such as, for instance, borates or chromates.

The temperature range of the electrolyte may extend from about 40 to about 100 C.

The applied voltage may be varied between limits of about 2 to about 20 volts. For this part direct current is to be used, which term is intended to include unidirectional current of any kind..

For the second part of the treatment the electrolyte may comprise, in addition to the aqueous solution of sodium bisulphate, aqueous solutions of such substances as sulphuric acid, chromium sulphate, chromium oxalate, chromic acid, boric acid, phosphoric acid, acetic acid or salts of such acids. The percentage of sodium bisulphate may be from 5 to 40 per cent. by weight, the temperature may be from 20-50 C. and the voltage from about 5 to about 30 volts. For this part the current may be of a direct or alternating character.

The film, presumably of aluminum oxide, produced by the second part of the process hereinbefore described, viz. the protective coating for the brightened surface is relatively hard but is quite transparent and differs from similar oxide films formed in other, e. g., sulphuric acid or oxalic acid, electrolytes in that it does not reduce the reflectivity value of the film produced in the first part of the process, in fact, in many instances it has been found that the reflectivity value has been slightly raised.

The process according to the invention is applicable to all kinds of surfaces either normal, polished or mat and in all cases results in a considerable increase of the specular reflectivity of the original material.

Whilst the foregoing particular example deals only with commercially pure rolled aluminum sheet, the scope of the invention is not limited thereto and aluminum alloys are included in the materials which are susceptible to beneficial treatment by the process according to the invention.

I claim:

1. A process for improving the reflectivity of the surface of a clean aluminum or aluminum alloy surface which comprises immersing the surface for a short time in an aqueous solution of sodium carbonate and tri-sodium orthophosphate in the proportions of about 3 parts by weight of the carbonate to about 1 part by weight of the phosphate and having a pH value of 10 or more and a temperature of about to C., and then without removing the surface from the solution applying direct current anodic treatment thereto.

2. A process as claimed in claim 1, including the further steps of washing the surface and providing a protective coating therefor. v

3. A process for improving the reflectivity of the surface of a clean aluminum or aluminum alloy surface which comprises immersing the surface for a short time in an aqueous solution of sodium carbonate and tri-sodium orthophosphate in the proportions of about 3 parts by weight of the carbonate to about 1 part by weight of the phosphate and having a pH value of 10 or more and a temperature of about 75 to 85 C., then washing the surface and providing a protective coating therefor by anodic treatment in an aqueous solution of sodium bisulphate.

4. A process as claimed in claim 3 wherein the sodium bisulphate solution contains from 5% to 40% by weight of the salt, is used at temperatures ranging from 20 C. to 50 C., and the current which may be direct or alternating in character is applied for about 10 to 15 minutes at a density of the order of 5 amperes per square foot of bright surface.

5. A process for improving the reflectivity of the surface of a clean aluminum or aluminum alloy surface which comprises immersing the surface for about 10 seconds in an aqueous solution of sodium carbonate and tri-sodium orthophosphate in the proportions of about 3 parts by weight of the carbonate to about 1 part by weightof the phosphate and having a pH value of 10 or more and a temperature of about 75 to 85 C. and then without removing the surface from the solution applying direct current anodic treatment thereto for about 10 to 15 minutes at a constant voltage which results in an initial current density of about 30 amperes per square foot of anode surface, then washing the surface and providing a protective coating therefor by anodic treatment in an aqueous solution of sodium bisulphate containing from 5% to 40% by weight of the bisulphate, the current, which may be alternating or direct in character being applied for about 10 to 15 minutes at a density of the order of 5 amperes per square foot of bright surface.

6. A process for improving the reflectivity of aluminum or aluminum alloy surfaces which comprises applying direct current anodic treatment in an electrolyte containing tri-sodium orthophosphate and sodium carbonate and having a pH value of 10 or more, immediately following a short simple immersion in the said electrolyte.

7. A process as claimed in claim 6 followed by rinsing and applying further anodic treatment with current of a direct or alternating character in an acid electrolyte containing from 5% to 40% sodium bisulphate.

8. A process as claimed in claim 6 wherein the period of the immersion treatment is of the order of 10 seconds and the alkaline electrolyte has an ortho-phosphoric radicle content approximately equal to that contained in from 3 to 9 per cent by weight of sodium ortho-phosphate.

9. A process for improving the reflectivity of aluminum or aluminum alloy surfaces which comprises immersing the surface in an electrolyte containing 15 per cent by weight of sodium carbonate and 5 per cent by weight of tri-sodium ortho-phosphate and having a temperature of from 75 to 85 C., after about 10 seconds immersion applying direct current anodic treatment for 10 to 15 minutes at a constant voltage which results in an initial current density of about 30 amperes per square foot of anode surface, and without interrupting the current withdrawing the anodic surface from the solution, then rinsing it in water and subjecting it to anodic treatment in a bath containing from 5 per cent to 40 per cent by weight of sodium bisulphate at a temperature ranging from 20 C. to 50 C. with current of a direct or alternating character for 10 to 15 minutes at a density of the order of 5 amperes per square foot of bright surface.

ALFRED NORMAN DOUGLAS PULLEN.

' and specified as me British Aluminium Company Limited, of London, Eng- CERTIFICATE CORRECTION. Patent No. 2,096,509. s October 1 9 1957. ALFRED NORMAN DOUGLAS.IU'. L ILEH,

It is hereby certified that the name e1 the assignee in the above numbered patent was erroneously deseribed and specified as "The British Aluminum Company Limited whereas said name should have been described land, as shown by the record of assignments in this office; and that the said Letters Patent should be read with this orrectioh therein that the ssmemay; conform to the record of the case in the Patent Office. Signed and sealed this 25th day of January, A. n. 1938.

I fienry Van Arsdale, (Seal) Acting Cemmissioner of Patents. 

